The model for active Na transport proposed by Koefoed-Johnsen and Ussing (1958) has been utilized for many years as a prototype for active transport in numerous epithelia. However, this model requires extensive modifications to be consistent with more recent experimental data and, as a result, fundamental questions have been asked about its basic assumptions: 1) The entry step, postulated in this model to be passive diffusion, may not be passive and it certainly does not exhibit simple Na diffusion. 2) Na may be compartmentalized in the cytosol and even bound, rather than uniformly dissolved in the cytosol. 3) The entry step and the serosal exit step may be linked together, instead of being independent steps. An experimental program with two separate approaches is proposed to obtain information regarding these questions. Kinetic transport studies will be performed in the presence of environments and reagents known to be site specific. In addition, fractionation of the frog skin will be attempted with the aim of isolating components with transport-related properties. It is hoped that these approaches will bring about a better understanding of epithelial transport systems at the molecular level, including energy conversion processes. Another portion of this project will attempt to separate the contributions of transcellular from those of paracellular transepithelial movement of water. The relationship between these two pathways is belived to be important in the control of water balance in many epithelia. The results of these experiments are expected to result in physical and mathematical models for the mechanism of action of active and passive transport in frog skin. The implications for the understanding of similar processes in other epithelia are readily apparent. BIBLIOGRAPHIC REFERENCE: Benos, D., S.A. Simon, L.J. Mandel, and P.M. Cala. 1976. The effect of amiloride and some of its analogues on cation transort in isolated frog skin and thin lipid membranes. J. Gen. Physiol. 68: 43-63.